This invention relates to the removal of gas from ink in an ink jet system and, more particularly, to a new and improved method and apparatus for removing dissolved gas from ink in an ink jet system.
In the copending application of Hoisington et al., application Ser. No. 043,372 filed Apr. 28, 1987, U.S. Pat. No. 4,788,556, a method and apparatus for removing dissolved air from ink are disclosed. As described in that application, in many ink jet systems ink is supplied to a chamber or passage connected to an orifice from which the ink is ejected drop by drop as a result of successive cycles of decreased and increased pressure applied to the ink in the passage. If the ink introduced into the passage contains dissolved air or other gas, decompression of the ink during reduced-pressure portions of the pressure cycle may cause the dissolved gas to form small bubbles in the ink within the passage. Repeated decompression of the ink in the chamber causes these bubbles to grow and such bubbles can produce malfunction of the ink jet apparatus.
In order to overcome this difficulty in accordance with the above-identified copending application, the ink in the ink jet system is subjected to a reduced pressure applied through a membrane which is permeable to gas but not to ink. To apply the reduced pressure, the ink is conveyed to an ink jet head through a passage which communicates through a permeable membrane with a plenum maintained at a reduced air pressure. As described in that application, the membrane may be a flexible sheet material such as, for example, a 0.01-inch thick layer of medical-grade silicone sheeting, such as Dow-Corning SSFMEXD-174, now available from Dow-Corning under the designation "2174". It has been found, however, that certain types of gas-permeable, ink-impermeable sheet material, such as silicone sheeting, tend to have a small but finite permeability to certain low-molecular-weight components of the ink, permitting small amounts of such components to pass through the membrane and accumulate as a scum on the low-pressure side of the membrane. If the scum-like accumulation is not removed periodically, it will reduce the efficiency of the membrane in the gas removal process.